<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>

<body>
    <input type="button" value="设置" onclick="f1()"> <br>
    <select id="fruits">
        <option>111111</option>
        <option>222222</option>
        <option id="xj">333333</option>
        <option>444444</option>
    </select>
    <script>
        function f1() {
            var xj = document.getElementById("xj");

            xj.selected = true;
        }
    </script>
</body>

</html>